1. Field of the Invention
The present invention relates to a droplet ejecting head and a droplet ejecting apparatus, in particular relates to the droplet ejecting head and the droplet ejecting apparatus, which eject a droplet to record characters and images on a recording medium or form a fine pattern, a thin film, and the like on a substrate.
2. Description of the Related Art
A method of ejecting an ink droplet is generally well known, which method including the steps of: generating a pressure wave (acoustic wave) by using means for generating pressure such as a piezoelectric actuator to liquid filled in a pressure generating chamber; and ejecting a liquid droplet from a nozzle communicated with the pressure generating chamber, by the pressure wave. Particularly, inkjet recording apparatuses which eject an ink droplet to record characters and images on a sheet of recording paper have become widesperad (for example, patent reference 1 and patent reference 2 described below). In recent years, inkjet recording apparatus can record extremely high-quality images, as a result of a decrease in an ink droplet volume and use of low-density ink.
Further, in recent years, several attempts have been made to utilize a droplet ejecting apparatus adopting the above-described droplet ejecting method in an industrial environment. Representative examples of such industrial utilization of a droplet ejecting apparatus include:    (a) forming lead patterns or transistors by ejecting an electrically conductive polymer solution on a substrate;    (b) forming an EL (electroluminescent) display panel by ejecting an organic EL solution on a substrate;    (c) forming a bump for electrical mounting by ejecting melted solder on a board;    (d) forming a three-dimensional object by laminating and curing a droplet of UV curable resin and the like on a substrate; and    (e) forming an organic thin film by ejecting a solution of an organic material (e.g., a solution of resist) on a substrate.
Thus, the application of a droplet ejecting apparatus is not limited to use for recording images. The droplet ejecting apparatus may be utilized in a variety of fields and it is expected that the field to which the droplet ejecting apparatus can be applied will further be extended in future.
Hereinafter an object on which a droplet is ejected with the droplet ejecting head will be referred to as “recording medium” and a pattern of dots which is obtained on a recording medium by depositing a droplet on the recording medium will be referred to as “image” or “recording image”. Therefore, “recording medium” in the following description includes not only recording paper and an OHP sheet but also a substrate as described above. “Image” in the following description includes not only general images such as characters, drawings, and photographs but also the above-mentioned lead pattern, three-dimensional object, and an organic thin film.
FIG. 24 is a sectional view showing an example of a droplet ejecting mechanism (an ejector) in a droplet ejecting apparatus well known in the patent references 1 and 2 described above. A pressure generating chamber 14 is coupled to a nozzle 16 for ejecting a droplet and a feed channel 20 for introducing liquid from a liquid tank (not shown) through a common channel 18 to the pressure generating chamber 14. A diaphragm 22 is provided on a bottom surface of the pressure generating chamber 14. When a droplet is to be ejected, a pressure wave is generated in the pressure generating chamber 14 by: displacing the diaphragm 22 by using a piezoelectric actuator 24 provided on a side of the diaphragm 22 which is opposite to the pressure generating chamber 14; and generating a change in a volume in the pressure generating chamber 14. A portion of the liquid filled in the pressure generating chamber 14 is injected toward the outside through the nozzle 16 by the pressure wave, to become a droplet 26, which then flies. The flying droplet 26 lands on a recording medium such as recording paper and the dot (image) is formed thereon. The pattern of characters and images is recorded (formed) on the recording medium by repeating the formation of the dot on the basis of image data and the like.
Currently, in the droplet ejecting apparatus as described above, improvement of the recording speed has been a major task. In a droplet ejecting apparatus, the parameter which most significantly affects the recording speed is the number of nozzles. The larger the number of nozzles is, the more the number of dots which can be formed per unit time is increased, and a higher recording speed is resulted. Therefore, a conventional droplet ejecting apparatus generally employs a multi-nozzle type droplet ejecting head (linear nozzle arrangement head) in which the plurality of ejectors are coupled to one another.
FIG. 25 shows a linear nozzle arrangement head 32 as an example of the multi-nozzle type droplet ejecting head. In the linear nozzle arrangement head 32, the liquid tank (not shown) is coupled to a common channel 36 through a liquid feed aperture 34 and the common channel 36 is coupled to a plurality of ejectors 38.
However, in the structure of FIG. 25 in which the ejectors 38 are arranged in one-dimensional manner (linearly), the number of ejectors cannot be increased so much (about 100 ejectors is the upper limit, normally).
Thus, there have been proposed several types of droplet ejecting head in which the number of ejectors is increased by arranging the ejectors in the form of a two-dimensional matrix (which type of droplet ejecting head will be referred to as “matrix-arrangement head” hereinafter) (refer to patent reference 3, patent reference 4 described below).
FIGS. 26A and 27A each show an example of a basic structure of the conventional matrix-arrangement head.
In the matrix-arrangement heads 42 and 52, a plurality of ejectors 44 are coupled to one another by each common channel 46, and a plurality of the common channels 46 are linked by a second common channel 48. In the matrix-arrangement head 42 shown in FIG. 26A, the common channel 46 is arranged along a main scanning direction of the head (indicated by an arrow M) and the second common channel 48 is arranged along a direction orthogonal to the main scanning direction (i.e., a sub-scanning direction, indicated by an arrow S). Each of ejectors 44A to 44H coupled to the same common channel 46 is arranged to be shifted by Pn in the sub-scanning direction. Dots 50 having a pitch Pn as shown in FIG. 26B are formed by ejecting a droplet from each ejector, while controlling ejection timing of each ejector in the process of scanning the head in the main scanning direction.
In the matrix-arrangement head 52 shown in FIG. 27A, the common channel 46 is arranged along the sub-scanning direction and the second common channel 48 is arranged along the main scanning direction. In this case, the ejectors adjacent to each other in the main scanning direction are also arranged to be each shifted by Pn in the sub-scanning direction. The dots 50 having the pitch Pn as shown in FIG. 27B are formed by ejecting a droplet from each ejector, while controlling the ejection timing of each ejector in the process of scanning the head in the main scanning direction.
In the matrix-arrangement head having the above-mentioned structure, it is easy to increase the number of ejectors, which is very advantageous in performing image recording at high speed. For example, in the matrix-arrangement head 42 shown in FIG. 26A, the 260 ejectors can be arranged by setting the number of common channels 46 to 26 and coupling ten ejectors 44 to each common channel 46 (In FIG. 26A, the number of common channels 46 is set to 8, the number of ejectors 44 per one common channel is set to 8, and only 64 ejectors 44 are shown, as a whole).
However, in the conventional matrix-arrangement head as described above, while the matrix-arrangement head has the advantage of high-speed recording, there is a problem that high uniformity of recording result is not easily obtained. Specifically, there is a problem that cyclic density unevenness (unevenness of a dot diameter) is easily generated in the direction (sub-scanning direction) orthogonal to the main direction of the head, which results in large loss of the uniformity of the recording result.
There are various reasons why such density unevenness is easily generated in the matrix-arrangement head. In particular, a change in ejection characteristics of the ejector (for example, droplet volume and ejecting speed of droplet) depending on a position of the ejector on a nozzle surface often results in the density unevenness.
In general, it is impossible to manufacture a head which is free of variations in the ejection characteristics of the ejector, and the farther the two ejectors are physically separated from each other, the larger the magnitude of variations in the ejection characteristics of the ejector. For example, in the case where the head is manufactured by laminating a member such as the substrate, deviation in a rotational direction among the laminated members results in the variations in the ejection characteristics among the ejectors. FIGS. 28A to 28D show examples of a case in which a positional deviation has been generated between the pressure generating chamber and the piezoelectric actuator. In the example shown in FIG. 28A, the pressure generating chamber 14 is formed by sandwiching a pressure generating chamber plate 54, in which a hole 56 is formed, from both sides with a diaphragm 58 and a nozzle plate 60. The pressure generating chamber 14 is disposed on one side of the diaphragm 58 and a piezoelectric actuator plate 62 is disposed on the other side of the diaphragm 58. A piezoelectric actuator 64 of the piezoelectric actuator plate 62 vibrates the diaphragm 58 to increase/decrease the volume of the pressure generating chamber 14 (see FIG. 28C), whereby a droplet is ejected from the nozzle (not shown). Accordingly, it is preferable that relative positions of the pressure generating chamber 14 with respect to the diaphragm 58 are the same in all the pressure generating chambers 14.
However, in practice, as shown in FIG. 28B, a deviation in the rotational direction may be generated between the pressure generating chamber plate 54 and the piezoelectric actuator plate 62, when the head is viewed from a direction perpendicular to the plate. As can be seen from FIG. 28B, the more downstream in the direction of the arrow S the pressure generating chamber 14 is located, the less area of the piezoelectric actuator 64 overlaps the pressure generating chamber 14. When the pressure generating chamber 14A at one end in the arrow S direction is compared with the pressure generating chamber 14B at the other end, the area of the pressure generating chamber 14B overlapped by the corresponding piezoelectric actuator 64 is less than the area of the pressure generating chamber 14A overlapped by the corresponding piezoelectric actuator 64.
Both of FIGS. 28C and 28D show action of the piezoelectric actuator 64 in the pressure generating chambers 14A and 14B. The diaphragm 58 is significantly deformed in the pressure generating chamber 14A in which the area thereof overlapped by the piezoelectric actuator 64 is relatively large. On the other hand, in the pressure generating chamber 14B in which the area thereof overlapped by the piezoelectric actuator 64 is relatively small, a portion of the piezoelectric actuator 64 also overlaps the pressure generating chamber plate 54 which is rigid (see a portion indicated by a circular two-dot chain line C1) and the deformation of the diaphragm 58 is constrained. That is, the amount of overlap between the pressure generating chamber 14 and the piezoelectric actuator 64 has an influence on the deformation of the diaphragm 58 and thus changes the ejection characteristics of the ejector. In the structure shown in FIG. 28B, the amount of overlap between the pressure generating chamber 14 and the piezoelectric actuator 64 is linearly changed according to the line of the ejectors. Therefore, the difference in the ejection characteristics between the ejectors is changed depending on a distance, along the line of the ejectors, from a reference position.
In addition to the deviation in the rotational direction, there also exist some factors of generating a difference in the ejection characteristics, depending on a distance along the line of the ejectors from a reference position. For example, positioning accuracy in the forming process of the nozzle is one of the factors. In order to eliminate variations in the ejection characteristics, it is necessary to accurately position the nozzle relative to the ejector in the forming process of the nozzle. The factors of the positioning accuracy include a difference in a scale between a machining apparatus and the matrix-arrangement head and the deviation in the rotational direction of the machining apparatus and the matrix-arrangement head. When such deviations are generated, the deviation of a nozzle position relative to the ejector is increased as the distance along the line of the ejectors increases, which results in a change in the ejection characteristics. Hereinafter, the linear change in ejection characteristics depending on the position of the ejector will be referred to as “linear ejection characteristics distribution.”
In the matrix-arrangement head, since the ejectors are arranged in the main scanning direction, as well, the linear ejection characteristics distribution may also be generated in the main scanning direction. When the recording is performed with the matrix-arrangement head having the linear ejection characteristics distribution in the main scanning direction, a change in the dot diameter having a cycle n is generated in the line of the recorded dots, as shown in FIGS. 26B and 27B. That is to say, the density unevenness having the cycle n in the sub-scanning direction is generated in the recording result.
In a general matrix-arrangement head, in order to realize recording of the resolution in a range from about 150 to about 600 dpi (dots per inch) in the sub-scanning direction, a nozzle pitch Pn ranges from 42.3 μm to 169.3 μm. This arrangement is generally realized with a matrix-nozzle arrangement whose n value is in a range of 4 to 20, approximately. However, in this arrangement, n tends to be increased in order to realize the narrower nozzle pitch. As a result, the cycle of the density unevenness is in a range of 0.42 to 3.4 mm, approximately, in practice. In other words, the density unevenness is generated with a spatial frequency in a range of 0.3 to 2.4 lines/mm.
FIG. 29 is a graph showing human visual sensitivity for density unevenness, in which graph the horizontal axis indicates the spatial frequency. It is found from FIG. 29 that, when the spatial frequency of the density unevenness is not more than 4 lines/mm, the human visual sensitivity for the density unevenness is increased and thus the density unevenness is easily perceived. In particular, when the spatial frequency of the density unevenness is not more than 3 lines/mm, density unevenness is very easily perceived. For the spatial frequency not more than 1 line/mm, there are two different data, i.e. the data that the sensitivity is decreased (broken line) and the data that the sensitivity is not decreased (solid line). According to experimental results by the inventors, the solid line represents the fact observed in practice better.
With reference to the human visual characteristics as described above, it is understood that the density unevenness of the spatial frequency ranging from 0.3 to 2.4 lines/mm which is generated in the conventional matrix-arrangement head is the one which is very easily perceived by human eyes and thus is likely to significantly mar the quality of the recording result. In order make the density unevenness less recognizable, it is necessary to set the spatial frequency of the density unevenness no less than 4 lines/mm or so, more preferably no less than 10 lines/mm or so. However, in the conventional multi-nozzle arrangement head, it is difficult to set the spatial frequency of the density unevenness in the above-described range. That is, highly uniform recording cannot be achieved with the conventional multi-nozzle arrangement head.
[Patent Reference 1]
    Japanese Patent Publication (JP-B) No. 53-12138[Patent Reference 2]    Japanese Patent Application Laid-Open (JP-A) No. 10-193587[Patent Reference 3]    Japanese Patent Application Laid-Open (JP-A) No. 1-208146[Patent Reference 4]    Japanese Patent Application Laid-Open (JP-A) No. 9-156095